1. Field of the Invention
The present invention relates generally to identifying substances (gases, liquids, or solids), and in particular, to using a holographic filter in a detection apparatus to detect a particular substance.
2. Description of the Related Art
Spectroscopy (such as infrared absorption spectroscopy [IR], or Raman spectroscopy) is a method often used to detect and identify substances (e.g., gases, liquids, or solids) such as toxic or explosive materials. To identify an unknown substance, the spectra (e.g., the wavelength and intensity) of light (that has been absorbed, emitted, or scattered) from the molecules of the unknown substance are measured. In this regard, the spectra of light provide a “fingerprint” that can be used to identify the molecules. Filters may also be used to separate different parts of the spectra by absorbing or reflecting certain wavelengths and transmitting other wavelengths. However, prior art spectroscopy and spectroscopic filters may not provide sufficient sensitivity for detecting a particular substance. These problems may be better understood by describing prior art spectroscopy and filters.
Spectroscopy utilizes the absorption, emission, or scattering of electromagnetic radiation by atoms or molecules (or atomic or molecular icons) to qualitatively or quantitatively study the atoms or molecules, or to study physical processes. To measure spectral reflectance, a variety of different types of spectrometers may be used. In this regard, spectrometers often record a spectrum on a detector at a focal plane after a light ray/beam proceeds through a series of lenses, apertures, stops, and diffraction gratings.
For example, a light source may initially be condensed and passed through an aperture and/or a collimating lens. A dispersing or diffracting element processes the light for collector optics (e.g., an imaging lens) that focus the light/spectrum onto a detector within a focal plane where the light may be recorded for subsequent processing (e.g., using a computer). Alternatively, the light beam may be processed through a collimating lens after which a beam splitter splits the light beam into multiple beams. Mirrors may then be utilized in combination with an imaging lens to project the beam onto a detector in the focal plane where the beam may be recorded. Thereafter, the information recorded on the detector may be obtained (e.g., transmitted or retrieved) by a computer for detailed analysis. An example of such a spectrometer is a Michelson interferometer.
To more accurately identify a substance, one or more filters may be used to transmit or reflect a specified range of wavelength. In this regard, the filter may be utilized with the imaging lens so that only desired wavelengths are recorded on the detector.
FIG. 1 illustrates the use of a Michelson interferometer and a filter in the prior art. As illustrated, a substance is illuminated with light at a point source 102 and is condensed using a condenser lens 104. The condensed light may then pass through slit 106 after which it is collimated using collimator lens 108. A beam splitter 110 is used to split the collimated light beam into two beams. One beam is reflected by the beamsplitter 110 to a plane fixed mirror 112. The other beam is reflected at 90 degrees to a movable plane mirror 114.
Both mirrors 112 and 114 reflect their respective beams back to the beamsplitter 110 and strike the beamsplitter 110 at the original incident beam's position. The beamsplitter 110 reflects the two beams to filter 116 where certain spectra may be filtered. The filtered light may then be focused using image-formation lens 118. The focusing lens 118 focuses the filtered light to provide an interference pattern at the focal plane where a detector 120 may be used to record the interference pattern. The interference pattern on the detector 120 may then be processed/analyzed using a computer to identify and detect the substance.
While the interferometer of FIG. 1 (and other interferometers or spectrometers) may be used to identify substances, it is often desirable to find ways to more accurately filter or more easily identify a substance. However, prior art methods and filters used for identifying substances may have a limited dynamic range among other disadvantages.